The goal of this study is to obtain antisera to products of genes carried by specific human chromosomes by immunizing mice or rabbits with mouse-human hybrid cells containing either a single human chromosome or a group of human chromosomes. The rabbit antisera will be absorbed exhaustively with mouse cells; the mouse antisera, collected at intervals after immunization of mice of the same strain as the hybrid parent, should require no absorption. The presence of antibodies directed against human gene products will be assayed with human cells, using an enzyme immunofluorescence technique in addition to antibody-antibody precipitation. Antisera containing such antibodies will be tested against a series of mouse-human hybrids and the results correlated with banded karyotype analysis to determine if the antisera provide a reliable way of detecting specific human chromosomes in interspecific hybrids. Immunoselection, using an antiserum in the presence of complement, will be tried to see if it provides a means of selecting for loss of a specific human chromosome in hybrid cells.